In addition to biological treatment, physical and chemical methods are also used for removal of colored dye substances in wastewater (Groff and Kim, 1989). In fact, the latter two have been more widely adopted for their effectiveness. Chemical methods often involve coagulation of dye substances followed by precipitation of the chemical sludge or oxidation process using ozone. Physical methods involve mainly adsorption by activated carbon or its like. In hybrid physical/chemical or physical/biological processes, ionizing irradiation and ultrafiltration are useful methods of pretreatment. Nevertheless, both physical and chemical methods have their shortcomings. Coagulation produces excess amount of chemical sludge and creates problem of its disposal. Oxidation employs costly ozone and is not effective for reductive or sulphur dye wastewater. Activated carbon also incurs high operating expenses and additional capital on activity regeneration. These are obvious opportunities and challenges for biological treatment method to offer viable alternatives in treating dye-containing wastewater.
Bio-decolorization of lignin-containing pulp and paper wastewater using white-rot fungi Phanerochaete Chrysosporium and Tinctporia sp. (Eaton, et al., 1980; Fukuzumi, et al., 1980) were clear examples of color removal thru microbial degradation of the colored substance, i.e., highly chlorinated and oxidized polymeric lignin molecules. Similar observation was reported later using another white-rot fungus Schizophyllum commune to decolorize wastewater from a bagasse pulping plant (Belsare, et al., 1988). As for dye color removal, a recent review (Groff and kim, 1989) described the ability of Rhodococcus, Bacillus cereus and Plesiomonas/Achromobacter to degrade soluble dyes, acid red dye and five azo-dyes, respectively. The widely practiced biological activated sludge method may be useful in removing COD and BOD in dye wastewater. They found little information concerning its effectiveness in dye color removal. On the other hand, textile dyes were found strongly adsorbed and held by wastewater treatment plant sludges that were landfilled. This suggests that adsorption may play another key role in biodecolorization.
Molasses wastewater may present an even tougher color removal task. One culture possessing this ability was found belonging to the Basidiomycetes family (Hongo, et al., 1973). Further screening utilizing melanoidin, the major colored substance in molasses, isolated Coriolus sp. 20 and found its color removing activity may be associated with the enzyme sorbose oxidase (Watanabe, et al., 1982). Subsequent investigation by Ohmomo, et al., at University of Tsukuba, Japan, yielded Coriolus versicolor Ps4a (Ohmomo, et al., 1985), Mycelia sterilia D90, Aspergillus fumigatus G-2-6, A. oryzae Y-2-32, and Lactobacillus hilgaridii W-NS, all found capable in decolorizing molasses.
Above reviewed bio-decolorization reports limit their studies primarily in defined laboratory model systems. Their actual application and effectiveness toward colored high strength industrial wastewater were not particularly emphasized. Though limited, successful examples of bio-decolorization of pulp and paper wastewater (U.S. Pat. No. 4,655,926, Chang, et al., 1987) and molasses wastewater (Sirianuntapiboon, et al., 1988) were reported. The former demonstrated the use of a rotating biological contacter and strains of white-rot fungi to remove color in waste liquor without giving quantitative results. While the latter claimed color removal of 40 percent or higher.